This invention relates to electric conductors incorporating optical fibres for communication purposes, and more especially (though not exclusively) to uninsulated conductors for use as earth conductors, or if desired as phase conductors, in an overhead electric power line. For brevity, such conductors will be referred to herein by the customary abbreviation xe2x80x9cOPGWxe2x80x9d (representing, less than precisely, optical ground wire).
The first successful OPGW designs (such as that introduced by our predecessors under the trademark FIBRAL) used fibres, or fibre ribbons, loosely housed in an aluminium tube formed around them by deforming an initially U-shaped extrusion, and generally filled with a waterproofing gel. Subsequently other designs have become established and even preferred by some suppliers.
More recently, a technique has become readily available for enclosing optical fibres in a welded metal tube (usually stainless steel) of relatively small cross-sectionxe2x80x94more specifically, a few millimetres in diameter and a fraction of a millimetre in wall thickness. Such sub-units are commonly referred to as xe2x80x9cfibre in metal tubexe2x80x9d (or xe2x80x9cfibre in steel tubexe2x80x9d), and this may be abbreviated to xe2x80x9cFIMTxe2x80x9d. FIMT can be incorporated among the wires to be stranded around a central core wire, but this subjects the FIMT to torsional manipulation during cable manufacture, which may risk degradation of the fibres and mechanical integrity of the tube, (and it is considered necessary to have a layer of plain wires over the layer containing an FIMT, for mechanical protection during and after installation). If the FIMT is being stranded around a core wire in this way, its diameter must be the same as or slightly smaller than that of the metal wires between which it is positioned. This restriction is one of the reasons why it is desirable to position the FIMT axially and strand plain wires around it. This, however, constrains conductor design significantly as the diameter and wall thickness of FIMT cannot be easily adjusted, and in particular it is difficult to achieve an overall diameter greater than about 6 mm, and this small diameter may demand the use of fewer and/or smaller wires in the layer immediately over the FIMT than would otherwise be desired, and in some cases may require an additional layer of wires, at significantly greater manufacturing cost.
It is therefore desirable to be able to increase the effective overall diameter of an FIMT economically and preferably to increase the overall conductance per unit area of the conductor.
JP 07-302519 describes an electric conductor comprising optical fibres and intended to provide an air-tight enclosure round the cables that exhibits long-term stability. It comprises a central axial element with one layer of elements helically stranded about it, and the central element comprises optical fibres enclosed in a longitudinally extending welded metal tube itself enclosed in a second metal tube. The first tube may be of copper or stainless steel and the second of aluminium.
In accordance with the invention, an OPGW or other electric conductor incorporating at least one optical fibre comprises a central axial element with at least one layer of elements helically stranded about it, at least one of the elements comprising at least one optical fibre enclosed in a longitudinally extending welded metal tube itself enclosed in a second metal tube of higher conductivity than the welded metal tube and is characterised in that the said second metal tube is of greater thickness than the said welded metal tube and has an unwelded longitudinal seam.
If there is only one element containing an optical fibre, it is preferably the central axial one.
The welded tube is preferably of stainless steel because of its good corrosion resistance and relatively easy welding; the second tube is preferably of aluminium (or a high-conductivity dispersion-strengthened aluminium alloy if designed for long spans) for good conductivity and low densityxe2x80x94in special cases copper might possibly be an alternative, for example if the conductor will be exposed to severely corrosive atmosphere.
The other elements will normally be plain wires which may similarly be of aluminium, aluminium alloy or copper but may also be of aluminium-clad (or copper-clad) steel, or some (but not normally all) of them may be of galvanised steel contributing little but tensile strength.
Optionally the longitudinal seam of the second metal tube may be sealed and/or the second metal tube may be bonded to the welded metal tube using a non-metallic adhesive, for better corrosion-resistance.
The second tube is preferably formed by first extruding a U-section (preferably comprising a semicircular base and straight limbs) and after inserting the FIMT wrapping its limbs to abut or nearly abut each other; the edges of the limbs may be shaped to interlock when engaged. The U-section is preferably extruded by the CONFORM technique (for which machinery is commercially available, for example from Holton Machinery Ltd) because it operates continuously and avoids possible property discontinuities that might arise if conventional ram extrusion were used. Hydrostatic extrusion may also be suitable in some cases. At least if the wall thickness of the second tube is not much greater than that of the welded tube, it may be a viable alternative to form it by wrapping an initially flat metal strip.